"Tibi" seems like a dative of agent, used with the gerundive to show on whom the necessity of the verbal idea rests (233, Allen and Greenough). The gerundive with "esse" is the most frequent use of the form, and it can be used as an impersonal verb with an object (315).

I would roughly translate it as, "You ought to sail the wide seas for a long time."

If the sentence were written with neuter plural, it would be agreeing with "maria", making "navigare" a passive idea that is attributive to the noun. "The vast seas that ought to be sailed by you for a long time."

Adrianus, I would have thought that in the context it would mean a long time - it can mean both 'for a long time' and 'by day', right?. It's about the wanderings of Aeneas etc...

Gaius,

I'm not sure if I follow your explanation: I went and looked at an online edition of Allen and Greenough and here's what I found:

The Dative of the Agent is used with the Gerundive to denote the person on whom the necessity rests: -haec vóbís próvincia est défendenda (Manil. 14), this province is for you to defend (to be defended by you). But this very example suggests that the gerundive 'defendenda' agrees with 'provincia', precisely not what occurs in my original sentence

The example doesn't clear up my confusion about the answer to the question: what case is 'navigandum'? and why doesn't it agree with 'maria'?

'Civibus patria defendenda est Patria remains in the nominative, and the gerundive still agrees with it. But the agent is in the dative. If you like you can think of this as a dative of interest (so far as the citizens are concerned...). This dative needn't be mentioned, as we saw above. '

I'm assuming that Vasta Maria is neuter nominative plural. 'tibi' is, of course, the dative and my basic question: Why is 'navigandum'? used where every single example I've found suggests it should be 'naviganda', remains.

Does not dative of agent merely, in this context, take a regular gerundive construction and simply ascribe responsibility with a dative pronoun or noun but otherwise leave it unchanged? So if, for example, I wanted to say Carthage must be destroyed and it's your responsibility to do it would I say:

BUT are you saying there are two things going on.....a dative of agent and an impersonal (indirect?) statement - meaning 'navigandum' is masculine accusative singular but, unless I've not been paying attention ( a distinct possibility) I haven't seen anything like this in LLPSI do date and Orberg doesn't explain it...?

"Navigandum" is a neuter sg., which is why I think it is impersonal (a gerundive construction). (I use "impersonal" because it was in the A. and G. grammar, but it also relates that there is no subject of "navigandum". An indirect sentence would be oratio obliqua I think.) The verbal idea doesn't agree with anything else because it is substantive. I think the dative of agent is a good explanation for "tibi" because the verb/subject is kind of mixed with "navigandum est" (A sailing ought to be done); someone has to be doing the sailing. "Vasta maria" seems like the object of what should be sailed.

The neuter formation of the gerundive may be used with trans. or intrans. verbs to form an impersonal sentence with "esse" (A. and G., 315). Here is one of the examples: Tempori serviendum est = One must obey the time. "Tempori" is the abl. object of "servire"; add a dat. of agent and switch the abl. object for a regular acc. and it would look like the sentence you posted.

"Vasta maria" is an object of "navigare". Diu, vasta maria tibi navigandum est = Sailing (a substantive, sg. adj. and a verbal idea) the wide seas (object) must be done (necessity implied in the gerundive) by you by day. It seems to me that the difference would be slight, but it would be more passive in tone if the sentence were: Diu, vasta maria naviganda sunt tibi = The wide seas ought to be sailed by you by day. "Naviganda" would just be attributive instead of impersonal.

I hope that makes sense. I have never read Orberg, so I don't know how or when he would introduce that idea to you.

I thought that the gerund only occurred in the dat., gen., acc., and abl. cases (316). Also: "When the Gerund would have an object in the Accusative, the Gerundive is generally used instead" (ibid). I assumed gerundives worked as adjectives also (only used when they agree with a noun or pronoun), but perhaps it can be a substantive gerundive in this case (which maybe would define it is a gerund?) or A. and G. are wrong that gerunds don't occur in the nominative case.

"Navigandum est" is certainly a gerundive of obligation. The gerundive has to agree with the passive subject, or, if it is impersonal, as it is in this case, it becomes neuter singular to match the implied subject. Looking at the text, I think you missed a preposition: from the online copy at amazon, the line reads:

Diu per vasta maria tibi navigandum est.

Which makes a lot more sense. Without looking it up, I'm not sure if "navigo" typically takes a direct object, and if so, what is the nature of the d.o. -- i.e. do you navigas a ship, or do you navigas the sea? (Or either?)

Which makes a lot more sense. Without looking it up, I'm not sure if "navigo" typically takes a direct object, and if so, what is the nature of the d.o. -- i.e. do you navigas a ship, or do you navigas the sea? (Or either?)